The hormonal mechanisms responsible for growth of the fetus are poorly understood. Most of the anabolic hormones known to stimulate post-natal growth have been shown to exert little or no effect on fetal growth. Only insulin and certain insulin-like peptides have been shown to have effects on fetal growth which might be of physiological importance. The purpose of this project is to test the hypothesis that somatomedin C is a primary stimulator of fetal growth. The specific objectives are: 1) to study the role of somatomedin C, insulin, and other anabolic peptides on the stimulation of growth of fetal tissues in organ culture; 2) to determine radioimmunoassayable levels of somatomedin C in fetal and cord serum and to examine by radioimmunoassay whether growth hormone, prolactin, or placental lactogen have the capacity to induce the formation of somatomedin C in the fetus and young animal; 3) to develop a practical method for quantitating the somatomedin binding protein and to measure this protein in serum; 4) to determine whether there is maternal-to-fetal or fetal-to-maternal transfer of somatomedin C.